Do Giant Cookiecutter Sharks Exist?
by Ben S. Roesch

Call me anti-climactic, but no, there probably aren't any giant cookiecutters out there. But it is an interesting story, and highlights the fascinating cookiecutter sharks (Isistius sp.) and a related species, the sleeper sharks (Somniosus sp.).

There are two known species of cookiecutter sharks: the cookiecutter shark (Isistius brasiliensis) and the largetooth cookiecutter shark (Isistius plutodus). Both are about the same size--reaching lengths of around 50 cm--and both belong to a family of deep water squaloid (Squaliformes--the dogfish sharks) sharks: Dalatiidae (the sleeper sharks belong to this family as well). The two Isistius species are epi- to mesopelagic creatures; it is thought that during the day they remain in the mesopelagic zone (from 200 m to 1000 m below sea level) and then at night they venture nearer to the surface. This behaviour is known as diel (influenced by day-night changes) vertical migration, and many other mesopelagic creatures, from fishes to shrimps, perform this behaviour.

Cookiecutters are also interesting in that they emit a luminous green glow from photophores on their bellies. This is likely used to attract the attention of the cookiecutters' prey: large fishes, pinnipeds and even whales and dolphins, which feed on luminous fishes and squids in the deep sea. When its prey is close enough, the cookiecutter commences its unique attack, by which it got its name. The cookiecutter clamps onto its prey's flesh with its jaws and bites down with its huge, sharp teeth on its lower jaw (the upper jaw teeth are much smaller and finer, but still very sharp, and are used to remain attached to the prey). It is believed that the cookiecutter then twists its body right around, gouging out a plug-like piece of flesh (sometimes the piece may have a thickness of twice the diameter of the mouth). The shark then may create oral suction with its thick fleshy lips, large tongue and strong throat muscles to hoover the piece of flesh out of the prey's body. Tunas, elephant seals, dolphins, whales, swordfish and other large marine animals have all been found with large gouges in their flesh (which do not pose any serious problem to the animals, which are much larger than the cookiecutter shark), and all evidence points to the cookiecutter shark as the culprit, especially since plugs of fish and marine mammal flesh have been recovered from the species' stomach. (Cookiecutters also feed on smaller fishes and squids, in which case the above "cookie-cutting" behaviour is not necessary and the cookiecutter will dispatch the prey like any other shark).

Mouth and teeth of cookiecutter shark (Isistius brasiliensis
All photos reproduced with permission from Australian Museum Online

Lateral view of cookiecutter shark head.

Body view of cookiecutter shark

But what of this "giant cookiecutter shark"? Several years ago I attended an excellent slide lecture show at the Royal Ontario Museum (in my hometown of Toronto) presented by the renowned Eugenie Clark. It was entitled "Sea Monsters and Other Mysteries of the Deep" or some such name.

After talking about such interesting things as the New Zealand 1977 "sea monster" carcass (really a rotten basking shark), weird abyssal octopuses, and whale sharks, she mentioned briefly a little story she heard from a colleague who had been working in Alaska.

One night, a dead narwhal was pulled it up along side the research boat, so that the researchers could examine it in the morning. It sat in the water all night, and in the morning as the scientists began to examine it, they found round bites on the animal, which strongly resembled those left by the cookiecutter shark (see below). However, these bites were much bigger than the ones usually made by known cookiecutters, which grow to a maximum of about 50 cm.

If my memory serves me correctly, Dr. Clark toyed, perhaps not too seriously, with the idea that there may be larger species of cookicutter sharks inhabiting Arctic waters. This is a fun idea, but there is an eminently more practical solution. Richard Martin, a marine biologist in Vancouver, Canada, wrote to me:

The most likely perpetrator of those 'large, cookie-cutter-like bites' is the Pacific sleeper shark (Somniosus pacificus), which frequently scavenges marine mammal carcasses; similar bites, which look as though they had been removed with a razor-edged icecream scoop, have been documented by Randall Reeves, Fred Bruemmer, and others on narwhals off Greenland, where the Greenland shark (Somniosus microcephalus) was demonstrated to be responsible. Given the limited palatoquadrate protrusion abilities of most squaloids, it is difficult to imagine how these huge dalatiinine sharks produce such perfectly circular bites - but hey, isn't that typical of sharks: to do what we didn't think they could?

Similar bites on seals washed up on Sable Island, Nova Scotia, are thought by some to have been caused by the greenland shark (Somniosus microcephalus) (see Chris McGowan's excellent popular guide to predator and prey in the natural world, The Raptor and the Lamb). Furthermore, sleeper sharks are closely related to the cookiecutter sharks as they reside in the same family, Dalatiidae, so there biting styles and appearance of bites may be similar.

Some might think I'd be disappointed that there is no giant cookiecutter behind Dr. Clark's story. For me, however, the known species are fabulous enough.